Furnace Exit Gas Temp Calc

[teaspirit]

All,
I’m trying to calculate a rough furnace exit gas temperature (FEGT). Does the following calculation make sense?

FEGT = ((Total Boiler Heat Input - Furnace Heat Input) + (Boiler Air Flow * Specific Heat Capacity of Air * (Air Heater Outlet Temp - Air Heater Inlet Temp)))/(Boiler Exit Gas Flow * Flue Gas Heat Capacity)

Any input is appreciated. Thank you.

 

[vpl]

I believe your equation is wrong.

It looks like you are trying to use the expression: Q = [ṁ]c_p(T1 - T2) to find the exit temperture. In order to do this, I believe you need at least an approximate furnace inlet temperature.

Remember that Q1 is generally equal to Q2 so your total boiler heat input(Q_b) = total furnace heat input (Q_f). Therefore, as written, your first term goes to 0.

If you take the relationship Q_b = Q_f and the relationship Q = [ṁ]c_p(T1 - T2)and combine them, you get [ṁ]_bc_pb(T_out,b - T_{in, b}) = [ṁ]_fc_pf(T_in,f - T_{out, f})

Rearranging and solving for T_{out, f} you get T_{out, f} = T_{in, f} + [ṁ]_bc_pb(T_out,b - T_{in, b}) /[ṁ]_fc_pf

or putting it in words: the furnace exit gas temperature is equal to the furnace inlet gas temperature plus the boiler air flow times the air specific heat capacity times the difference between the boiler air outlet temperature and the boiler air inlet temperature divided by the furnace gas flow times the gas specific heat capacity.


Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[teaspirit]

Sorry, let me clarify. When I say "Furnace Heat Input" I mean the heat from just the furnace, which excludes heat from the superheater, reheater, and economizer. I assumed that Boiler Heat Input = Furnace HI + Superheater HI + Reheater HI + Economizer HI. Was that a safe assumption? Have I missed heat input from somewhere else in the boiler that should be taken into account? Thanks again.

 

[FOURe]

In my opinion, the fuel and oxidant composition needs to be defined as well as the fuel and oxidant inlet temperature values and combustion conditions in order to calculate the combustion products flame temperature.

Once the flame temperature of the combustion products is known, the combustion products exit temperature could be calculated based upon the assumption of knowing how much heat gets transfered to the steam streams.

Here is a URL where you can find a spreadsheet to help you out calculate the combustion products flame temperature for your combustion conditions:

http://www.engineering-4e.com/eng-tips/combustionwithplots.xls

Once the combustion products flame temperature is known, one can use the combustion products specific heat capacity in order to calculate the combustion products exit temperature as state in the above posts ...

The above should be sufficient to estimate the combustion products exit temperature.

PS

Fuel, oxidant (combustion stoichiometry), steam mass flow rates and some steam inlet and outlet temperature, pressure and enthalpy values need to be defined to put the puzzle together ...

http://www.engineering-4e.com

 

[FOURe]

Here are two plots that might be helpful in determining the combustion products flame and exit temperature values:

​

http://www.engineering-4e.com

 

[davefitz]

I recommend you use the "russian normative method" as defined by a.g.blokh in "heat transfer in in steam boiler furnaces" and also as summarized in sect 8.3.2 of s.Kakc's text " boilers evaporators and condensers"- very simple and is accurate once you have as background the FEGT vs heat input of a similar furnace firing a similar fuel. without this backgroun you can use the standard factors listed in these texts.

 

[FOURe]

Here is another useful URL provided by eFunda.com regarding the steam tables calculations to help you with your calculations:

http://www.efunda.com/materials/water/steamtable_general.cfm

http://www.engineering-4e.com

 

[athomas236]

The Furnace Exit Gas Temperature (FEGT) can be calculated by a heat balance on the furnace assuming that the temperatures in the furnace are uniform and equal to the FEGT.

The heat balance is on a per lb of fuel basis where the:

Heat input is the sum of the heat in the fuel (NCV BTU/lb) and the heat in any hot combustion air entering the furnace.

The heat in hot air is the weight of air per lb of fuel times the mean specific heat of air (Typically 0.24 BTU/lb F) times (air temperature F minus Reference temperature (usually 77F))

Heat output is the heat transferred by radiation to the furnace walls plus the heat in the gases leaving the furnace.

The transferred by radiation per lb of fuel is

0.1723 times furnace area (ft^2) times furnace factor times (((FEGT F+460)/100)^4-((Twall F +460)/)^4) divided by fuel flow lb/h. Furnace factor is typically 0.85 for coal and oil firing.

The heat in gas leaving the furnace is the weight of gas per lb of fuel times the mean specific heat of gas (Typically 0.28 BTU/lb F) times (FEGT F minus Reference temperature (usually 77F))

The heat balance is solve by trial and error for FEGT.

Regards,

athomas236